Covering approximately 295,000 miles[11] between eight states and one Canadian province, the Great Lakes-St. Lawrence River system includes Lakes Superior, Michigan, Huron[12], Erie and Ontario and their connecting channels.Graphic courtesy of the U.S. Department of the Interior.

The recent decline of water levels across the Great Lakes has generated considerable media coverage and alarming pronouncements about climate change. But a new report by researchers with the U.S. Geological Survey documents a natural variation[1] in Great Lakes levels throughout millenniums. Moreover, the fluctuations of the past century have been far less dramatic than previous ones. The geologic record also reveals thousands of years of significant climate variability in the Great Lakes region, according to the report, which is excerpted below.

The hydrograph above depicts water levels for Lake Michigan-Huron over the past 5,000 years, from 3,000 BC to the present. The upper x-axis refers to the entire 5,000-year cycle measured, while the lower x-axis refers to actual calendar years. The y-axis at left measures water levels in meters, and in feet at right.Graphic courtesy of the U.S. Department of the Interior.

Water levels in the lakes vary naturally on timescales that
range from hours to thousands of years. Short-term changes are triggered by
storms and seiches[2]; seasonal changes are driven by differences in snowmelt,
precipitation and evaporation; annual to millennial changes are driven by subtle to major climatic changes affecting both precipitation (and resulting stream
flow) and evaporation. Rebounding of the Earth’s surface in response to loss of
the weight of melted glaciers has also affected water levels.

Recorded Water-Level History

Dredging, control structures[3] , locks, dams, hydroelectric facilities, canals and diversions[4] have altered the hydrology of the Great
Lakes/St. Lawrence River System. Dredging and control structures have had the
largest impacts. For instance, dredging of the St. Clair River from 1880 to 1965
permanently lowered Lake Michigan/Huron by about 16 inches. Control structures
at the outlets of Lake Superior and Lake Ontario keep the levels of these lakes
regulated within a range that is smaller than the range of levels that would
occur under natural outflow conditions.

Recorded lake-level histories for each lake show some
similarities. Periods of higher lake levels generally occurred in the
late-1800s, the late-1920s, the mid-1950s, and from the early-1970s to
mid-1980s. Pronounced low lake levels occurred in the mid-1920s, the mid-1930s
and the mid-1960s, and returned again in 1999. Because Lake Superior water
levels have been regulated since about 1914 and levels of Lake Ontario have been regulated since about
1960, lake-level patterns on those lakes since regulation began do not reflect
all the natural variability that would have occurred without regulation. For
example, unregulated Lakes Michigan/Huron and Erie had extremely high
water-level peaks in 1929, 1952, 1973, 1986 and 1997, as well as extreme lows
bottoming out in 1926, 1934, 1964 and 2003. Some of those extreme levels,
especially the lows, were muted in Lakes Superior and Ontario after regulation
began.

Reconstructed Water-Level History

The Great Lakes are rimmed by coastal features[5] and associated
sedimentary deposits, some as old as 14,000 years and some that are developing
today. Many of these features are formed by and respond to changes in lake
level.[6] … Because many … coastal deposits formed in response to either short-term
or long-term fluctuations in lake level, they can be used to reconstruct
lake-level changes that preceded instrument measurement of water levels that
began in the mid-1800s ... .

For the Lake Michigan Basin, data from five strandplains[7] were combined to
produce a
hydrograph[8] of lake-level change over the past 4,700 years.[9] ... [Several] periodic
lake-level fluctuations were active in the past and are probably still active in
the lake basin today. ... [The] lake level was roughly 13 feet higher 4,500 years
ago. This high phase is called the Nipissing II phase[10] of ancestral Lake
Michigan, and it is represented around the lakes by high, dune-capped ridges,
mainland-attached beaches, barrier beaches and spits. This shoreline commonly
was instrumental in isolating small lakes from the larger lake basins. The
Nipissing II phase was followed by more than 500 years of lake-level decline
during which lake levels dropped to elevations similar to historical averages. Three high phases from 2,300 to 3,300, 1,100 to
2,000, and zero to 800 years ago followed this rapid decline. Pervasive in the
hydrograph is a quasi-periodic[13] rise-and-fall pattern of about 160 ± 40 years in
duration. This fluctuation can be extended into the historical record, and it
appears that the entire historical dataset (mid-1800s to present) may be one
such 160-year quasi-periodic fluctuation. Superimposed on this 160-year
fluctuation is a short-term fluctuation of 32 ± 6 years in duration. This
lake-level rise-and-fall pattern produced the individual beach ridges in most
embayments[14] and is also expressed in the historical data, most easily seen in the
low levels in the 1930s and 1960s and again starting in the late 1990s.

Relation to Climate

... The importance of climate variability in controlling Great
Lakes water levels during the past 5,000 years has been assessed by comparing
independent proxy records[15] of past climate variability with the reconstructed
water-level history of Lake Michigan inferred from sediments. The development of
high-resolution and well-dated paleoclimate[16] records, such as those from inland
bogs and lake sediments, has revealed significant climatic variability in the
Great Lakes region at decadal to millennial timescales during the past several
thousand years. One unsettling pattern in these records is that, despite being a
relatively humid region, severe droughts larger than any observed in the past
century occurred several times in the last few thousand years and had large and
long-lasting ecological effects. For example, between about 1,000 and 700 years
ago, a time interval broadly consistent with the Medieval Warm Period,[17] a series
of large-magnitude moisture fluctuations occurred over the western Great Lakes
region, the Great Plains and the western United States.[18] Lake Michigan water
levels were greatly affected by these fluctuations, particularly a large drought
about 1,050 years ago. This large drought dramatically altered forest
composition in southeastern Michigan[19] and may have extended well into eastern
North America.

Another major drought in the region, which was probably even larger than the
Medieval Warm Period droughts, was associated with the large drop in Lake
Michigan water levels between 4,500 and 4,000 years ago. At that time, water
levels in Lake Michigan dropped at a rate at least five times the rate of
isostatic rebound.[20] Although non-climatic factors may have been involved in this
rapid drop, the timing corresponds to a well-documented and widespread
centennial-scale drought that affected much of the North American mid-continent
— activating dune systems, causing widespread fires and leading to long-lasting changes in forest composition.[21] Abrupt climate
changes at that time are well documented on most continents, suggesting
potential global-scale linkages.

Times of prolonged high water levels in the Great Lakes (highstands) have
also been linked to climate variability. For example, bog surface-moisture
reconstructions and inland lake records from throughout the Great Lakes region
indicate wetter conditions during highstands.[22] Pollen records indicate that
populations of trees favoring moist conditions also expanded at these times.[23]
Although some climate changes associated with lake-level fluctuations were
widespread, others were probably more spatially variable, with different areas
of the Great Lakes Basin receiving more or less moisture. The water-level
history of the Great Lakes integrates these spatial patterns. Comparison of
localized records of climate variability from throughout the Great Lakes Basin
(for example, records from small lakes, bogs and tree rings) with the regionally integrated record of Great
Lakes water-level history will … help develop hypotheses regarding the
atmospheric-circulation patterns associated with Great Lakes water-level
fluctuations at scales of decades to millennia.

Clearly, the water balance of the Great Lakes region has
varied considerably, and the overall variability for the past 14,000 years far
surpasses that of the last 100 years in magnitude and ecological effect.
Mechanisms behind climatic variability at these long timescales are poorly
understood; however, many severe moisture fluctuations of the past century have
been linked to dynamics of the ocean-atmosphere system, particularly variability
in sea-surface temperatures and the associated changes in atmospheric
circulation. For example, sea-surface temperature variability in both the
Pacific and the Atlantic has been linked to changes in atmospheric circulation
that influence the water balance of the mid-continent, including the Great Lakes
region.[24] Interactions between land surface and atmosphere, particularly with
regard to soil moisture, often extend and amplify a large drought.[25] The extreme
fluctuations in water balance evident in the Great Lakes water-level history and
other paleoclimatic records may represent interactions and amplifications of
this kind, as well as responses of the ocean-atmosphere system to variability in
external influences such as solar radiation and volcanic activity.[26] …

Relation to Coastal Ecosystems

Water-level fluctuations in the Great Lakes are of great
ecological importance in the coastal zone because even small changes in lake
level can shift large areas from being flooded to being exposed and vice versa.
The vegetation of shallow-water areas in the Great Lakes is the one biotic
resource most directly affected by natural or regulated changes in water level.
Individual plant species and communities of species have affinities and
physiological adaptations for certain water-depth ranges, and their life forms
may show adaptations for different water-depth environments. Changes in water
level add a dynamic aspect to the species-depth relation and result in shifting
mosaics of wetland vegetation types. In general, high water levels kill trees,
shrubs, and other emergent[27] vegetation, and low water levels following these
highs result in seed germination and growth of a multitude of species. Some
species are particularly well suited to recolonizing exposed areas during
low-water phases, and several emergents may coexist there because of their
diverse responses to natural disturbance.

In the first year after a reduction in water levels, the distribution of new
seedlings is due to the distribution of seeds in the sediments. In ensuing
years, the distribution of full-grown plants is due to survival of seedlings as
they compete for growing area. If one species is favored in early colonization,
its density may be great enoughthat it can maintain dominance of an area. In most cases,
early colonizing species or communities are later lost through competitive
displacement, but the opportunity to go through a life cycle allows them to
replenish the seed bank in the sediments. Occasional low water levels are also
needed to restrict growth of plants that require wet conditions, such as
cattails, at higher elevations in wetlands that are typically colonized by
sedges and grasses.

The magnitude of lake-level fluctuations is of obvious
importance to bordering wetland vegetation because it directly results in
different water-depth environments.[28] The different plant communities that develop
in a Great Lakes wetland shift from one location to another in response to
changes in water depth. The water-depth history largely determines the species
composition of a particular site at a given point in time…

The effect of water-level changes on shorelines varies with
the morphology, composition and dominant processes of the coast. Variability in
lake levels causes erosional and depositional processes to take place at
different elevations over time. The most dramatic effect is the impact of an
elevated storm surge during high lake levels, flooding low-lying areas and
eroding mobile substrates. These storms can liberate sediment from upland areas,
feeding the littoral[29] system, and can ultimately nourish downdrift shorelines.
The effects of this nourishment may not be seen until times of low water levels
when exposed sand bars, widened beaches and dune growth are evident.

Water-level fluctuations in the Great Lakes also play a major
role in development and stabilization of coastal dunes. Studies of buried soils
within dunes along the southeastern shore of Lake Superior and eastern shore of
Lake Michigan show that dune building occurred during the high lake-level
periods that have recurred about every 160 years. High lake levels destabilize
coastal bluffs and make sand available to leeward perched dunes.[30] Intervening
periods of lower lake levels and relative sand starvation permit forestation and
soil development on the dunes.[31]

Summary

... Independent investigations of past climate change in the
basin over the long-term period of record confirm that most of these changes in
lake level were responses to climatically driven changes in water balance,
including lake-level highstands commonly associated with cooler climatic
conditions and lows with warm climate periods. The mechanisms underlying these
large hydroclimatic anomalies are not clear, but they may be related to internal
dynamics of the ocean-atmosphere system or dynamical responses of the
ocean-atmosphere system to variability in solar radiation or volcanic activity.
...

The extreme high and low lake levels measured in recorded
lake-level history have altered storage by as much as 31 cubic miles in Lake
Michigan-Huron and as little as 9 cubic miles in Lake Ontario. Diversions of
water into and out of the lakes are very small compared to the total volume of
water stored in the lakes.

Conclusion

A variety of factors influence lake levels that may shift
hourly, daily and seasonally, or even over centuries and millennium. Some of the
causes are easily understood, including the levels of snowmelt, precipitation
and drought. Other possible causes are not fully understood, including the
impacts of solar and volcanic activity and the interaction between Earth’s
atmosphere and oceans. But the latest research by the U.S. Geological Survey
documents that current conditions are well within the natural variability of
long-term cyclical change.

[1] Excerpted from the U.S. Department of the Interior U.S. Geological Survey Circular 1311 “Lake-Level Variability and Water Availability in the Great Lakes”; available online at http://pubs.usgs.gov/circ/2007/1311/

[2] Stationary waves usually caused by strong winds and/or changes in barometric pressure. Seiches are found in lakes, semi-enclosed bodies of water and areas of the open ocean.

[12] Lakes Huron and Michigan are usually considered as one lake because of their wide connection at the Straits of Mackinac.

[13] A repetitive behavior that is not uniform in period or amplitude.

[14] A hyrdrogeological formation that either resembles or is actually a bay.

[15] A reconstructed history of environmental changes based on the contents of a natural archive (for example, sediments, ice cores), typically using an indicator, measurement, or suite of measurements that are highly correlated with a particular environmental variable (for example, temperature).